Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9.5-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
More recently, the FMCW principle has been improved, and today typically involves transmitting not a continuous sweep but a signal with stepped frequency but practically constant amplitude. An example of such a stepped FMCW is disclosed in U.S. Pat. No. 5,406,842. For a number of discrete frequencies a phase shift is determined, and based on a set of such phase shifts an IF signal is obtained, having the same properties as the continuous IF signal described above. In principle, this requires a number of frequencies, N, greater than a number stipulated by the sampling theorem. The distance to the reflecting surface is then determined using the frequency of the IF signal in a similar way as in a conventional FMCW system. Typical values can be 200-300 IF periods at 30 m distance divided in 1000-1500 steps.
Although highly accurate, FMCW systems are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell).
General Disclosure of the Invention
It is an object of the present invention to provide an improved method for radar level gauging, with lower power requirements than the conventional FMCW method.
According to a first aspect of the present invention, this and other objects are achieved by a method for measurement of a distance to a surface of a product kept in a tank.
The method comprises transmitting low power electromagnetic transmit signals towards the surface, receiving electromagnetic return signals reflected at the surface, determining the distance based on an initially estimated distance and a relationship between the transmit signals and the return signals. The transmit signals are formed as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, the pulse train has an average duty cycle of less than 50 percent, each pulse has a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The method further comprises determining actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determining, based on the initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlating the actual phase properties with the expected phase properties to provide an updated estimation of the distance.
The present invention is based on transmitting a set of carrier wave pulses, each having a distinct frequency selected within a frequency range. The method is therefore referred to as a Frequency Modulated Pulsed Wave (FMPW).
Unlike the method referred to above as stepped FMCW, the number of different carrier wave frequencies in a measurement cycle is insufficient to provide a continuous IF signal, or even an approximation of the IF frequency used in a conventional FMCW system. Instead, the small set of frequencies is chosen according to a specified frequency scheme, and a phase shift in the received pulse is determined for each frequency. The set of phase shifts allow a determination of a change compared to a previously recorded distance to the reflecting surface. In most applications the user requires an updating rate in the order of once per second and then the level change between two measurements is small.
As the pulses are emitted and processed distinctly, and independently of each other, the duty cycle of the transmit signal can be reduced, and is less than 50%. In some embodiments of the invention it is significantly less, and may be 5% or even 1% or lower. This means that individual pulses may be emitted with a relatively high power, without increasing the average power of the measurement cycle. This makes the present invention particularly useful in situations where power is scarce, such as in a field unit powered by an industrial current loop (e.g. 4-20 mA loop) or in a battery power (or solar powered, etc) field unit.
The correlating can include determining a distance offset based on the actual phase properties and said expected phase properties for each carrier wave frequency, and determining the updated estimation of the distance based on the initially estimated distance and the distance offset.
Alternatively, the correlating can include determining a relationship between actual phase and carrier wave frequency. Such a relationship may be expressed as a slope of a line in a diagram, which slope indicates the distance.
The order of the pulses with difference frequencies is not critical for the determination of the distance, and the pulses may for example be transmitted in ascending or descending order, or any mixed order.
The frequency scheme may be designed in various ways, and may include dividing an overall frequency range (e.g. in the order of GHz) into a number of subranges, and defining a subset of discrete frequencies for each subrange. The schedule may further include randomly selecting one frequency each from each frequency subset.
According to one embodiment of the invention, the schedule is adaptive, i.e. it may vary depending on the current measuring situation. For example, it may be advantageous to increase the number of pulses when more difficult measuring conditions are at hand. Such difficult measuring conditions may include a disturbed or quickly moving surface, and may present themselves as an increased uncertainty in the measurement result (quantified e.g. as a variance or standard deviation).
In this case, the schedule may include several accuracy levels, each including a different number of frequencies. When a measurement result acquired using one accuracy level becomes too uncertain, the accuracy level is increased, and a larger number of pulses with different frequencies are used in the next cycle.
Conventional FMCW with a linear or stepped sweep converts the set of echoes from various distances in the tank to a “tank spectrum”. The interesting echo may be filtered out to decrease disturbing echoes from other parts of the tank. This is not possible with the simplest type of MFPW according to the present invention. To improve the situation it may be advantageous to introduce a frequency modulation of the carrier wave, and to mix the received signal with the modulation frequency, in order to provide dependence on the distance. This dependence can be used to eliminate undesired echoes by low pass filtering.
The transmitter signal is preferably frequency modulated by a suitable frequency fm (several complete periods within each pulse), providing a variation of the carrier frequency within a frequency band δf in the order of MHz. With such modulation, two advantages are available:                1. The signal can be received around fm or a multiple of fm (instead of DC) which will make it less sensitive for 1/f noise and transient-like disturbances.        2. A distance dependence is introduced, which might be advantageous for instance to limit the echo competition to the nearest ±1 m etc.        
A second aspect of the present invention relates to a system for FMPW, comprising a transceiver for transmitting low power electromagnetic transmit signals and receiving electromagnetic return signals reflected at the surface, and processing circuitry for determining the distance based on an initially estimated distance and a relationship between the transmit signals and the return signals. The transceiver is arranged to form the transmit signals as a pulse train of distinct carrier wave pulses having a duration greater than 1 microseconds and shorter than 100 milliseconds, the pulse train having an average duty cycle of less than 50 percent, each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The processing circuitry is arranged to determine actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determine, based on the initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlate the actual phase properties with the expected phase properties to provide an updated estimation of the distance.
In order to minimize energy consumption in a Multiple Frequency Pulsed Wave (MFPW) radar level gauging system, as described above, it has been found to be advantageous to use a small number of frequencies for providing an updated estimation of the distance to the surface of the product in the tank. As indicated, several variations of this principle are possible. However, occasionally, a Frequency Modulated Continuous Wave (FMCW) frequency sweep measurement can be inserted to verify or correct the updated estimation of the distance obtained through MFPW distance measurement. Also, a new, initially estimated distance may be provided based on such FMCW measurement or on the updated estimation of the distance or a combination thereof. Further, the FMCW frequency sweeps for estimating the distance are advantageously inserted at different times that depend on the measurement situation or at predetermined time intervals. The measurement situation, as pertaining to echoes received from the tank, could be characterized as, for instance, still surface, slowly moving surface, rapidly moving surface, and existence of disturbing echoes (from structures inside tank at certain locations).
The above-described procedure of inserting FMCW frequency sweeps is applicable to both the method and the system aspects of the invention.